For the production of liquid metal, electric arc fusion furnaces (EAF) normally use the injection of a jet of oxygen combined with other gases (for example methane or particulates such as graphite) in order to remove, with an exothermic chemical reaction, undesired elements contained in the liquid metal bath.
The production of heat “in situ”, generated by this reaction, is therefore added to the energy of an electric origin, supplied by the electrodes. The effect of this combined action is accelerating the production cycle, at the same time improving the quality of the product (liquid metal). The benefits induced by this technique essentially depend on the effective quantity of gas or particulate materials made available to the liquid metal. This is obviously effected by limiting as much as possible, the dispersion of heat which directly influences the reaction kinetics in the bath. The technique adopted in the lances applied to EAFs envisages a De Laval nozzle specifically studied for allowing the jet of oxygen or other material, to reach supersonic velocities with a collimation sufficient for perforating the outer layer of slag and enabling the gas to be bubbled into the liquid bath.
Two types of lances have been developed for effecting the above principle. The first, from a historical point of view, is the so-called “door lance”. This lance is a few meters long and enters the furnace through the openings present therein, for example the slag door.
The accessibility is limited as not all points of the furnace can be reached, but a solution of this kind allows the thermal problem to be concentrated on the tip of the lance; all the other components are left outside the furnace in a relatively safe and easily accessible environment.
This solution obviously has various limitations induced by the accessibility to the furnace and especially the insufflation point of the oxygen; in order to homogenize the bath, the stirring must therefore be particularly efficient.
A further limitation consists in the necessity, for the whole time necessary for the insufflation, to keep the passage space of the lance open with the consequent entrance of false cold air and the introduction of vapors and heat.
This obviously jeopardizes the consumptions and environmental impact of the whole system.
It should finally be pointed out that these movable systems have considerable dimensions and costs (not only investment but also operating; it is sufficient to consider the water consumptions for guaranteeing the cooling of a lance 4 meters long) in proportion to these.
A further critical point consists in the fact that an arm coming from the outside can be damaged by scrap still stacked, in the collapsing or oscillating phase in the bath making its use under conditions other than those of a flat bath, unadvisable.
To try and overcome these problems, small-dimensioned and relatively inexpensive lances have been developed, of which an example is provided in the patent Valery SHVER CA2449774. The ejectors of these lances (as disclosed, for example, by the same Valery SHVER in U.S. Pat. No. 6,289,035 and U.S. Pat. No. 6,614,831) are arranged in a fixed position inside the furnace so as to insufflate oxygen into the bath, and also other technical substances (see, for example, the disclosures of Hubert KNAPP and Peter HOFFMAN in U.S. Pat. No. 5,332,199). Their geometrical arrangement in plan view (and their inclination) allows reactive elements and heat energy in the most critical positions (on the edges and in the areas farthest away from the electrodes) of the bath to be transferred, with the effect of homogenizing the refining of the metal, increasing the stirring and, in ultimate analysis, reducing the steelwork cycle times.
Positioning inside the furnace implies exposure to a particularly hostile environment. The weakest parts of the lance (for example the connection area with the gas lines and entry regulation valves) must be kept in a protected and cooled area, far from direct exposure to heat and spurts of slag coming from the bath. Practically all of the cited reference documents disclose that the lance must be positioned inside a cooled structure anchored to the wall of the furnace and passing through a bulkhead which supports them, constrains them and enables their disassembly and maintenance. This bulkhead also has the function of isolation preventing the atmosphere and heat of the furnace from being dispersed in the surrounding environment.
Arrangements of this kind obviously entail greater difficulty in the maintenance as it is necessary to operate with the furnace at a standstill and also because the lances are covered by layers of slag due to the spurts produced by the insufflation, stirring and introduction of materials into the furnace. Modern arc furnaces use a containment crucible of the melting material, partially cooled and partially refractory. In the most modern steel production techniques, this recipient can be substituted when worn by another identical crucible having a new refractory coating. This change of casing operation is effected by lifting with a crane and it is evident that the more rapidly this takes place, the more time there is for the production. It is obvious that the harder the slag encrustation, the longer the times necessary for the reconditioning of the lances will be, before their re-installation on the new crucible.
A further problem of the known arrangements consists in the irremovability of the same lances. Once they have been fixed to the supporting bulkhead, their distance from the metal only depends on the level reached by the liquid metal and by the overlying slag. Due to the necessity of effecting maintenance operations with the furnace at a standstill (therefore after long periods) and the fact that, when inactive (without any injection of gas or other material), there is no shield which prevents the deposit of slag on the nozzles, it is necessary to protect the lance as much as possible by removing it from the bath. In practice, even if fluid-dynamic systems are used for extending the jet coherence (such as the shrouding effect), their use can only be exploited under flat bath refinery conditions (i.e. when almost all the scrap charged has become liquid).
It is evident that this imposes serious limitations to the cycle times, inhibiting the possibility of operating with the bath refining contemporaneously with the dissolving of the scrap.
Again, the art has so far tried to overcome this state of affairs by acting on the very nature of the concept of the lance. In EP1848927 (of BIANCHI FERRI, MEMOLI, POZZI, MALFA) the lance, during the scrap dissolving phase, can operate as a burner modulating its emission intensity continuously and without mechanical movements, in relation to the specific necessities of the moment.
It is evident that this method substantially accelerates the dissolving phase, but for this very reason it cannot anticipate the beginning of the refining phase.
The same can be said for the lance-burner described in U.S. Pat. No. 4,752,330 and in U.S. Pat. No. 4,865,297 of Grigory GITMAN. This, in fact, is a group positioned on a wall of the furnace and oriented in the direction of the bath. A common oxygen tube, with the possibility of also operating in immersion, is supported in the body of the lance-burner. In this configuration, the oxygen passing through the tube is combined with the fuel injected into the fixed combustion chamber (anchored to the wall of the furnace) acting as burner (low impact energy of the outgoing gases and high heat transfer values to the scrap), until the dissolution of the charge. In the refining phase (without encumbrances of scrap), the oxygen tube extends through the body of the lance-burner reaching the bath and extremely high impact energy values of the gases typical of oxygen lances. The system therefore commutes from the burner configuration to the lance configuration without the possibility of intermediate steps. The system proposed is consequently not able to effect metal refining in the presence of scrap and a high efficiency and efficacy of use is only associated with the first moments of the steelwork cycle with a batch charge. When operating under a continuous charging condition of the scrap (for example with a Consteel®) or when the collapse of the charge removes the material from the oxygen pipe, the system, in a burner configuration, gradually becomes less efficient even if it cannot yet operate as a lance due to the presence of floating scrap in the bath.
Finally, from the inclination of the supporting system of the lance and maximum extension possible of the same, it can be deducted that even in the absence of scrap, this system would in any case not be able to operate for typical levels of liquid metal of a foot at the start of the cycle. Further critical points are associated with the investment costs (the combustion in fact develops inside the group making the requisites of the material used for its construction particularly critical) and maintenance (from what can be deducted from the patents, the group is assembled on the wall of the furnace without any protection, the lance tube which extends from this must be cooled for the whole of its extension and this requires the consumption of a large quantity of water or other cooling liquid).
It should be pointed out, however, that the above patents refer to an oxygen lance.